(1) Field of the Invention
This invention relates to a feedback control circuit of an LED driving circuit, and more particularly relates to the feedback control circuit of an LED driving circuit for enhancing LED diming accuracy.
(2) Description of the Prior Art
Because of the properties of long lifetime, high luminance efficiency, fast and steady illumination, and etc., LED has been broadly accepted as the main trend of light sources for the next generation in recent years. The LEDs can be used in various applications, including interior lighting, exterior lighting, and commercial advertisement lighting, and etc., and thus the existing light sources are gradually replaced by the LEDs. It is an important issue to have the LEDs generate illumination with steady brightness and uniform color and provide suitable protection to the LEDs to bring the lighting advantages of the LEDs into play.
FIG. 1 is a circuit diagram of a typical LED driving circuit. As shown, the LED driving circuit includes a feedback control circuit 100, a converter circuit 130, and an LED module 140. The converter circuit 130 is coupled to an input power source VIN. The feedback control circuit 100 generates a control signal Sc1 to control the conversion operation of the converter circuit 130. The output of the converter circuit 130 is coupled to the LED module 140 for applying an output voltage VOUT to the LED module 140 such that an output current IOUT corresponding to the output voltage VOUT is generated flowing through the LED module 140 to generate illumination. The output current IOUT also flows through a current sensing resistor Ri for generating a current feedback signal IFB1.
The feedback control circuit 100 includes a pulse width control unit 110 and a feedback unit 120. The feedback unit 120 has an amplify unit 122 and a compensation unit 124. The amplify unit 122 receives the current feedback signal IFB1 and a reference signal Vr and generates an output signal. The output signal is compensated by the compensation unit 124 so as to generate a pulse width control signal Vea1. The pulse width control unit 110 includes a pulse width modulation unit 112 and a driving unit 114. The pulse width modulation unit 112 receives the pulse width control signal Vea1 and a triangular signal so as to generate a pulse width modulation signal S1 to the driving unit 114. The driving unit 114 generates the control signal Sc1 according to the pulse width modulation signal S1.
Generally, the feedback control circuit 100 would be able to stabilize the output current IOUT at a predetermined current value Io such that the output voltage VOUT would be stabilized at a predetermined voltage value Vo also. However, the feedback control carried out by the amplify unit 122, which compares the current feedback signal IFB1 and the reference signal Vr, and the compensation unit 124, which compensates the error of the above two signal to adjust the level of the pulse width control signal, may generate the output current IOUT and the output voltage VOUT with the values close to the predetermined output current Io and the predetermined output voltage Vo.
FIG. 2 is a diagram of waveforms describing dimming process of the LED driving circuit in FIG. 1. The driving unit 114 receives a dimming signal DIM and according to the dimming signal DIM decides whether outputting the control signal Sc1 or not. In the time zone from T1 to T4, the dimming signal DIM implies ON state and the driving unit 114 outputs the control signal Sc1. In the time zone from T4 to T1, the dimming signal DIM implies OFF state and the driving unit stops outputting the control signal Sc1. In the time zone from T4 to T1, because the driving unit 114 stops outputting the control signal Sc1 to have the converter circuit 130 stopped transmitting power to the LED module 140, the level of the output voltage VOUT may decline to the lighting threshold voltage Vf of the LED module 140 at time point T5 and so the output current IOUT would be reduced to zero. In such condition, a positive error between the reference signal Vr and the current feedback signal IFB1 is generated to enhance the level of the pulse width control signal Vea1, even to the maximum level. The output voltage VOUT would be maintained at the lighting threshold voltage with no power loss in the time zone from T4 to T1 in ideal, however, leakage current is unpreventable in actual circuit, which may result in the decreasing of output voltage VOUT, even to the level below the lighting threshold voltage Vf in the time zone from T5 to T1. At time point T1, as the driving unit 144 retrieves the control signal Sc1, the duty cycle of the control signal Sc1 would be maximized for the level of the pulse width control signal Vea1 is at the maximum.
Then, after time point T2, the increasing output current IOUT is above the predetermined output current Io, the amplify unit 122 begins to lower the level of the pulse width control signal Vea1. However, because of the characteristics of the compensation unit 124, the pulse width control signal Vea1 cannot quickly decline to a stable level Vea1o, which is the level of the pulse width control signal Vea1 corresponded to the predetermined output current Io. At this time, the duty cycle of the control signal Sc1 is too large with respect to the output current IOUT. Thus, the output current IOUT keeps increasing until the pulse width control signal finally decreases below the stable level Vea1o. As the output current IOUT declines below the predetermined output current again, the level of the pulse width control signal Vea1 increases again and then to level above the stable level Vea1o. The above mentioned cycle repeats until time point T3, when the output current IOUT, the output voltage VOUT, and the pulse width control signal Vea1 converge to the predetermined output current Io, the predetermined output voltage Vo, and the stable level Vea1o. 
Thus, as the dimming signal indicates the OFF state corresponded to the time zone from T4 to T1, the output voltage VOUT may decline to the level below the lighting threshold voltage Vf because of leakage current, and as the dimming signal indicates the ON state corresponded to the time zone from T1 to T4, the LED driving circuit needs a long time before reaching stable state, which may influence dimming accuracy of the LED driving circuit.